Positive-electrode active material and battery

ABSTRACT

A positive-electrode active material contains a compound that has a crystal structure belonging to a space group FM3-M and that is represented by the composition formula (1): 
       Li x A y Me z O α F β   (1)
         wherein A denotes Na or K, Me denotes one or two or more elements selected from the group consisting of Mn, Co, Ni, Fe, Al, B, Ce, Si, Zr, Nb, Pr, Ti, W, Ge, Mo, Sn, Bi, Cu, Mg, Ca, Ba, Sr, Y, Zn, Ga, Er, La, Sm, Yb, V, and Cr, and the following conditions are satisfied.       

       1.7≦ x+y ≦2.2
 
       0≦ y ≦0.2
 
       0.8≦ z ≦1.3
 
       1≦α≦2.5
 
       0.5≦β≦2

BACKGROUND 1. Technical Field

The present disclosure relates to a positive-electrode active materialfor use in batteries and to a battery.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 2015-118892discloses a positive-electrode active material for use in lithiumsecondary batteries, the positive-electrode active material being alithium ion composite transition metal oxide with an α-NaFeO₂ structureand with a lithium/transition metal ratio of more than 1, thepositive-electrode active material containing sodium and potassium.

SUMMARY

In the related art, there is a demand for a battery with a high energydensity.

In one general aspect, the techniques disclosed here feature apositive-electrode active material according to one aspect of thepresent disclosure contains a compound that has a crystal structurebelonging to the space group FM3-M and that is represented by thecomposition formula (1).

Li_(x)A_(y)Me_(z)O_(α)F_(β)  (1)

A denotes Na or K, Me denotes one or two or more elements selected fromthe group consisting of Mn, Co, Ni, Fe, Al, B, Ce, Si, Zr, Nb, Pr, Ti,W, Ge, Mo, Sn, Bi, Cu, Mg, Ca, Ba, Sr, Y, Zn, Ga, Er, La, Sm, Yb, V, andCr, and the following conditions are satisfied: 1.7≦x+y≦2.2, 0≦y≦0.2,0.8≦z≦1.3, 1≦α≦2.5, 0.5≦β≦2.

The present disclosure can provide a battery with a high energy density.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a battery of a secondembodiment; and

FIG. 2 is an X-ray powder diffraction chart of a positive-electrodeactive material of Example 1.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described below.

First Embodiment

A positive-electrode active material of a first embodiment contains acompound that has a crystal structure belonging to the space group FM3-Mand that is represented by the composition formula (1).

Li_(x)A_(y)Me_(z)O_(α)F_(β)  (1)

A denotes Na or K,

Me denotes at least one selected from the group consisting of Mn, Co,Ni, Fe, Al, B, Ce, Si, Zr, Nb, Pr, Ti, W, Ge, Mo, Sn, Bi, Cu, Mg, Ca,Ba, Sr, Y, Zn, Ga, Er, La, Sm, Yb, V, and Cr (that is, one or two ormore elements selected from the group).

Furthermore, the following conditions are satisfied.

1.7≦x+y≦2.2

0<y≦0.2

0.8≦z≦1.3

1≦α≦2.5

0.5≦β≦2

Such an embodiment can provide a high-capacity battery with a highenergy density.

For example, a lithium-ion battery containing a positive-electrodeactive material containing the compound has an oxidation-reductionpotential (vs. Li/Li) of approximately 3.3 V. In the case that Me is Mn,the energy density is approximately 1065 mWh/g or more.

In the composition formula (1), x+y of less than 1.7 indicates that theamount of available Li in the compound decreases. Thus, the energydensity becomes insufficient.

In the composition formula (1), x+y of more than 2.2 (or z of less than0.8) indicates decreased utilization of an oxidation-reduction reactionof an available transition metal in the compound. This results inincreased utilization of an oxidation-reduction reaction of oxygen. Thisdestabilizes the crystal structure and results in an insufficient energydensity.

In the composition formula (1) of the compound, y=0 indicatesinsufficient crystal strain in the charging process and high resistanceduring the intercalation of Li. Thus, the energy density becomesinsufficient.

In the composition formula (1) of the compound, y of more than 0.2indicates slower diffusion of Li in the solid phase. Thus, the energydensity becomes insufficient.

In the composition formula (1), a of less than 1 (or 3 of more than 2)indicates that the effects of highly electronegative F on the compoundincrease. This results in decreased electron conductivity andinsufficient capacity.

In the composition formula (1), a of more than 2.5 (or 3 of less than0.5) indicates that highly electronegative F has a small influence onthe compound. This decreases cation-anion interaction. This destabilizesthe structure when Li is desorbed and therefore results in insufficientcapacity.

In the positive-electrode active material of the first embodiment, acompound represented by the composition formula (1) has a crystalstructure belonging to the space group FM3-M (rock-salt-type crystalstructure).

In the composition formula (1), the ratio of Li to Me is represented by{Li_(x)/Me_(z)}.

In the ratio, 1.7≦x+y≦2.2, 0<y≦0.2, and 0.8≦z≦1.3.

Thus, the ratio of Li to Me is theoretically in the range of1.15≦{Li_(x)/Me_(z)}≦2.75 and is more than 1.

The number of Li atoms per Me atom is larger than that for a knownpositive-electrode active material, for example, LiMnO₂.

In a compound represented by the composition formula (1), Li and Me areprobably located at the same site.

Thus, a compound represented by the composition formula (1) canintercalate and deintercalate more Li per Me atom than a knownpositive-electrode active material, for example, LiMnO₂.

Solid solution in which highly electronegative F is dissolved in ananion can improve the operating voltage.

Thus, the positive-electrode active material of the first embodiment issuitable for high-capacity lithium-ion batteries with a high energydensity.

Upon abstraction of much Li, a layered structure specified by the spacegroup R3-M cannot hold the layers and disintegrates.

By contrast, a rock-salt-type crystal structure specified by the spacegroup FM3-M, such as a compound according to the present disclosure, canstably maintain the structure without disintegration, even afterabstraction of much Li.

Solid solution of Na or K with a large ionic radius increases thelattice constant and improves the diffusion of Li ions. Thus, ahigh-capacity battery with a high energy density can be provided.

The positive-electrode active material of the first embodiment maycontain the compound as a main component.

Such an embodiment can provide a higher-capacity battery with a highenergy density.

The term “main component”, as used herein, means that the compoundconstitutes, for example, 90% or more by weight of thepositive-electrode active material of the first embodiment.

In addition to the compound as a main component, the positive-electrodeactive material of the first embodiment may contain incidentalimpurities, or starting materials for the synthesis of the compound,by-products, and degradation products.

In the positive-electrode active material of the first embodiment, thecompound may satisfy x+y+z=α+β=3 in the composition formula (1).

Such an embodiment can provide a higher-capacity battery with a highenergy density.

In the positive-electrode active material of the first embodiment, Me inthe composition formula (1) may be one element selected from Mn and Co.

Such an embodiment can provide a higher-capacity battery with a highenergy density.

In the positive-electrode active material of the first embodiment, thecompound may satisfy 0<y≦0.1 in the composition formula (1).

Such an embodiment can provide a battery with a higher energy density.

In the positive-electrode active material of the first embodiment, thecompound may satisfy 1.8≦x≦1.99 and 0.01≦y≦0.2 in the compositionformula (1).

Such an embodiment can provide a higher-capacity battery with a highenergy density.

In the positive-electrode active material of the first embodiment, thecompound may satisfy z=1, α=2, and β=1 in the composition formula (1).

Such an embodiment can provide a higher-capacity battery with a highenergy density.

In the positive-electrode active material of the first embodiment, Me inthe composition formula (1) may be one element selected from Mn, Co, Ni,and Fe, a solid solution composed of Ni, Co, and Mn, or a solid solutioncomposed of Ni, Co, and Al.

Such an embodiment can provide a higher-capacity battery.

In the positive-electrode active material of the first embodiment, thecompound may satisfy 1.79≦x+y≦2.18 in the composition formula (1).

Such an embodiment can provide a higher-capacity battery.

In the positive-electrode active material of the first embodiment, thecompound may satisfy 1.89≦x+y≦2 in the composition formula (1).

Such an embodiment can provide a higher-capacity battery.

In the positive-electrode active material of the first embodiment, thecompound may satisfy 0.79≦β≦1 in the composition formula (1).

Such an embodiment can provide a higher-capacity battery.

<Method for Producing Compound>

An exemplary method for producing the compound of the positive-electrodeactive material of the first embodiment will be described below.

A compound represented by the composition formula (1) can be produced bythe method described below, for example.

A raw material containing Li, a raw material containing A, a rawmaterial containing F, and a raw material containing Me are prepared.Examples of the raw material containing Li include oxides, such as Li₂Oand Li₂O₂, salts, such as LiF, Li₂CO₃, and LiOH, and lithium compositetransition metal oxides, such as LiMeO₂ and LiMe₂O₄. Examples of the rawmaterial containing A include AF and A2O. Examples of the raw materialcontaining F include LiF and transition metal fluorides. Examples of theraw material containing Me include oxides with various oxidation states,such as Me₂O₃, salts, such as MeCO₃ and MeNO₃, hydroxides, such asMe(OH)₂ and MeOOH, and lithium composite transition metal oxides, suchas LiMeO₂ and LiMe₂O₄. In the case that Me is Mn, examples of the rawmaterial containing Mn include manganese oxides with various oxidationstates, such as Mn₂O₃, salts, such as MnCO₃ and MnNO₃, hydroxides, suchas Mn(OH)₂ and MnOOH, and lithium composite transition metal oxides,such as LiMnO₂ and LiMn₂O₄.

These raw materials are weighed at the mole ratio of the compositionformula (1).

The variables “x, y, z, α, and β” in the composition formula (1) can bealtered in the ranges described for the composition formula (1).

The weighed raw materials are mixed, for example, by a dry process or awet process and are allowed to react mechanochemically for 10 hours ormore to produce a compound represented by the composition formula (1).For example, a mixing apparatus, such as a ball mill, may be used.

The raw materials and the conditions for mixing a mixture of the rawmaterials can be adjusted to produce a compound substantiallyrepresented by the composition formula (1).

The use of a lithium transition metal composite oxide as a precursor candecrease the energy for mixing elements. Thus, a compound represented bythe composition formula (1) can be produced with higher purity.

The composition of a compound represented by the composition formula (1)thus produced can be determined by ICP spectroscopy and an inert gasfusion-infrared absorption method, for example.

A compound represented by the composition formula (1) can be identifiedby determining the space group of the crystal structure by powder X-rayanalysis.

Thus, a method for producing a positive-electrode active materialaccording to one aspect of the first embodiment includes (a) a step ofpreparing the raw materials and (b) a step of mechanochemically reactingthe raw materials to produce the positive-electrode active material.

The step (a) may include a step of mixing a raw material containing Liand F and a raw material containing Me at a Li/Me mole ratio in therange of 1.31 to 2.33 to prepare a raw material mixture.

The step (a) may include a step of producing a raw material, a lithiumtransition metal composite oxide, by a known method.

The step (a) may include a step of mixing the raw materials at a Li/Memole ratio in the range of 1.7 to 2.0 to prepare a raw material mixture.

The step (b) may include a step of mechanochemically reacting the rawmaterials in a ball mill.

Thus, a compound represented by the composition formula (1) may besynthesized by mechanochemically reacting a precursor (for example, LiF,Li₂O, a transition metal oxide, a lithium composite transition metal,etc.) in a planetary ball mill.

The mixing ratio of the precursor can be adjusted to introduce more Liatoms.

When the precursor is reacted by a solid phase method, the precursor isdecomposed into more stable compounds.

Thus, a compound that has a crystal structure belonging to the spacegroup FM3-M and is represented by the composition formula (1) cannot beproduced by a method for reacting the precursor by a solid phase method.

Second Embodiment

A second embodiment will be described below. The contents described inthe first embodiment are appropriately omitted to avoid overlap.

A battery of the second embodiment includes a positive electrodecontaining the positive-electrode active material of the firstembodiment, a negative electrode, and an electrolyte.

Such an embodiment can provide a high-capacity battery with a highenergy density.

More specifically, as described in the first embodiment, thepositive-electrode active material contains many Li atoms per Me atom.Thus, a high-capacity battery can be provided.

As described in the first embodiment, the positive-electrode activematerial contains a compound containing a solid solution of Na or K.Thus, a battery with a high energy density can be provided.

The battery of the second embodiment may be a lithium-ion secondarybattery or a non-aqueous electrolyte secondary battery, for example.

In the battery of the second embodiment, for example, the negativeelectrode may contain a negative-electrode active material that canadsorb and desorb lithium (that has lithium adsorption and desorptioncharacteristics) or lithium metal.

In the battery of the second embodiment, for example, the electrolytemay be a non-aqueous electrolyte (for example, a non-aqueous electrolytesolution).

FIG. 1 is a schematic cross-sectional view of a battery 10 of the secondembodiment.

As illustrated in FIG. 1, the battery 10 includes a positive electrode21, a negative electrode 22, a separator 14, a case 11, a sealing plate15, and a gasket 18.

The separator 14 is disposed between the positive electrode 21 and thenegative electrode 22.

The positive electrode 21, the negative electrode 22, and the separator14 are impregnated with a non-aqueous electrolyte (for example, anon-aqueous electrolyte solution).

The positive electrode 21, the negative electrode 22, and the separator14 constitute an electrode group.

The electrode group is housed in the case 11.

The case 11 is sealed with the gasket 18 and the sealing plate 15.

The positive electrode 21 includes a positive-electrode currentcollector 12 and a positive-electrode active material layer 13 disposedon the positive-electrode current collector 12.

The positive-electrode current collector 12 is formed of a metallicmaterial (aluminum, stainless steel, aluminum alloy, etc.), for example.

The positive-electrode current collector 12 may be omitted, and the case11 may be used as a positive-electrode current collector.

The positive-electrode active material layer 13 contains thepositive-electrode active material of the first embodiment.

If necessary, the positive-electrode active material layer 13 maycontain an additive agent (electrically conductive agent, ionicconduction aid, binder, etc.).

The negative electrode 22 includes a negative-electrode currentcollector 16 and a negative-electrode active material layer 17 disposedon the negative-electrode current collector 16.

The negative-electrode current collector 16 is formed of a metallicmaterial (copper, nickel, aluminum, stainless steel, aluminum alloy,etc.), for example.

The negative-electrode current collector 16 may be omitted, and thesealing plate 15 may be used as a negative-electrode current collector.

The negative-electrode active material layer 17 contains anegative-electrode active material.

If necessary, the negative-electrode active material layer 17 maycontain an additive agent (electrically conductive agent, ionicconduction aid, binder, etc.).

The negative-electrode active material may be a metallic material,carbon material, oxide, nitride, tin compound, or silicon compound.

The metallic material may be a single metal or an alloy. Examples of themetallic material include lithium metals and lithium alloys.

Examples of the carbon material include natural graphite, coke, carbonunder graphitization, carbon fiber, spherical carbon, artificialgraphite, and amorphous carbon.

From the perspective of capacity density, silicon (Si), tin (Sn),silicon compounds, and tin compounds can be suitably used. Siliconcompounds and tin compounds may be alloys and solid solutions.

Examples of the silicon compounds include SiO_(x) (wherein 0.05<x<1.95).Compounds (alloys and solid solutions) produced by substituting anotherelement for part of silicon of SiO_(x) may also be used. The otherelement may be at least one selected from the group consisting of boron,magnesium, nickel, titanium, molybdenum, cobalt, calcium, chromium,copper, iron, manganese, niobium, tantalum, vanadium, tungsten, zinc,carbon, nitrogen, and tin.

Examples of the tin compound include Ni₂Sn₄, Mg₂Sn, SnO_(x) (wherein0<x<2), SnO₂, and SnSiO₃. One tin compound selected from these compoundsmay be used alone. Alternatively, two or more tin compounds selectedfrom these compounds may be used in combination.

The negative-electrode active material may have any shape. Thenegative-electrode active material may have a known shape (particulate,fibrous, etc.).

The negative-electrode active material layer 17 may be filled with(adsorb) lithium by any method. More specifically, the method may be (a)a method for depositing lithium on the negative-electrode activematerial layer 17 by a gas phase method, such as a vacuum evaporationmethod, or (b) a method for heating a lithium metal foil in contact withthe negative-electrode active material layer 17. In these methods,lithium can be diffused into the negative-electrode active materiallayer 17 by heat. Alternatively, lithium may be electrochemicallyadsorbed on the negative-electrode active material layer 17. Morespecifically, a battery is assembled from the negative electrode 22 freeof lithium and a lithium metal foil (positive electrode). Subsequently,the battery is charged to adsorb lithium on the negative electrode 22.

Examples of the binder for the positive electrode 21 and the negativeelectrode 22 include poly(vinylidene difluoride),polytetrafluoroethylene, polyethylene, polypropylene, aramid resin,polyamide, polyimide, polyamideimide, polyacrylonitrile, poly(acrylicacid), poly(methyl acrylate), poly(ethyl acrylate), poly(hexylacrylate), poly(methacrylic acid), poly(methyl methacrylate), poly(ethylmethacrylate), poly(hexyl methacrylate), poly(vinyl acetate),polyvinylpyrrolidone, polyether, polyethersulfone,hexafluoropolypropylene, styrene-butadiene rubber, andcarboxymethylcellulose. Other examples of the binder include copolymersof two or more materials selected from the group consisting oftetrafluoroethylene, hexafluoroethylene, hexafluoropropylene,perfluoroalkyl vinyl ether, vinylidene fluoride,chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene,fluoromethyl vinyl ether, acrylic acid, and hexadiene. The binder mayalso be a mixture of two or more materials selected from thesematerials.

Examples of the electrically conductive agent for the positive electrode21 and the negative electrode 22 include graphite, carbon black,electrically conductive fiber, graphite fluoride, metal powders,electrically conductive whiskers, electrically conductive metal oxides,and electrically conductive organic materials.

Examples of the graphite include natural graphite and artificialgraphite. Examples of the carbon black include acetylene black, ketjenblack (registered trademark), channel black, furnace black, lampblack,and thermal black. Examples of the metal powders include aluminumpowders. Examples of the electrically conductive whiskers include zincoxide whiskers and potassium titanate whiskers. Examples of theelectrically conductive metal oxides include titanium oxide. Examples ofthe electrically conductive organic materials include phenylenederivatives.

The separator 14 may be formed of a material that has high ionpermeability and sufficient mechanical strength. Examples of such amaterial include microporous thin films, woven fabrics, and nonwovenfabrics. More specifically, it is desirable that the separator 14 beformed of a polyolefin, such as polypropylene or polyethylene. Theseparator 14 formed of a polyolefin has not only good durability butalso a shutdown function in case of excessive heating. The separator 14has a thickness in the range of 10 to 300 μm (or 10 to 40 μm), forexample. The separator 14 may be a monolayer film formed of onematerial. Alternatively, the separator 14 may be a composite film (ormultilayer film) formed of two or more materials. The separator 14 has aporosity in the range of 30% to 70% (or 35% to 60%), for example. Theterm “porosity”, as used herein, refers to the volume ratio of pores tothe separator 14. The “porosity” is measured by a mercury intrusionmethod, for example.

The non-aqueous electrolyte solution contains a non-aqueous solvent anda lithium salt dissolved in the non-aqueous solvent.

Examples of the non-aqueous solvent include cyclic carbonate solvents,chain carbonate solvents, cyclic ether solvents, chain ether solvents,cyclic ester solvents, chain ester solvents, and fluorinated solvents.

Examples of the cyclic carbonate solvents include ethylene carbonate,propylene carbonate, and butylene carbonate.

Examples of the chain carbonate solvents include dimethyl carbonate,ethyl methyl carbonate, and diethyl carbonate.

Examples of the cyclic ether solvents include tetrahydrofuran,1,4-dioxane, and 1,3-dioxolane.

Examples of the chain ether solvents include 1,2-dimethoxyethane and1,2-diethoxyethane.

Examples of the cyclic ester solvent include γ-butyrolactone.

Examples of the chain ester solvents include methyl acetate.

Examples of the fluorinated solvents include fluoroethylene carbonate,methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate,and fluorodimethylene carbonate.

The non-aqueous solvent may be one non-aqueous solvent selected fromthese. Alternatively, the non-aqueous solvent may be a combination oftwo or more non-aqueous solvents selected from these.

The non-aqueous electrolyte solution may contain at least onefluorinated solvent selected from the group consisting of fluoroethylenecarbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methylcarbonate, and fluorodimethylene carbonate.

These fluorinated solvents in the non-aqueous electrolyte solutionimprove the oxidation resistance of the non-aqueous electrolytesolution.

Consequently, even when the battery 10 is charged at a high voltage, thebattery 10 can operate stably.

Examples of the lithium salt include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆,LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)(SO₂C₄F₉), andLiC(SO₂CF₃)₃. The lithium salt may be one lithium salt selected fromthese. Alternatively, the lithium salt may be a mixture of two or morelithium salts selected from these. The concentration of the lithium saltranges from 0.5 to 2 mol/liter, for example.

The battery of the second embodiment may be of various types, such ascoin type, cylindrical type, square or rectangular type, sheet type,button type, flat type, or layered type.

EXAMPLES Example 1 [Production of Positive-Electrode Active Material]

LiF, NaF, Li₂O, and Mn₂O₃ were weighed at a mole ratio ofLiF/NaF/Li₂O/Mn₂O₃=0.95/0.05/0.5/0.5.

The raw materials, together with a proper amount of 3-mm zirconia balls,were put in a 45-cc zirconia container, which was then sealed in anargon glove box.

It was removed from the argon glove box and was treated in a planetaryball mill at 600 rpm for 30 hours.

The resulting compound was subjected to X-ray powder diffractionmeasurement.

FIG. 2 shows the results.

The space group of the compound was FM3-M.

The composition of the compound was determined by ICP spectroscopy andinert gas fusion-infrared absorption.

The composition of the compound was Li_(1.9)Na_(0.1)MnO₂F.

[Production of Battery]

70 parts by mass of the compound, 20 parts by mass of an electricallyconductive agent, 10 parts by mass of poly(vinylidene difluoride)(PVDF), and a proper amount of 2-methylpyrrolidone (NMP) were mixed toprepare a positive-electrode mixture slurry.

The positive-electrode mixture slurry was applied to one side of apositive-electrode current collector formed of aluminum foil 20 μm inthickness.

The positive-electrode mixture slurry was dried and rolled to form apositive-electrode plate with a positive-electrode active materiallayer. The positive-electrode plate had a thickness of 60 μm.

A circular positive electrode 12.5 mm in diameter was punched out fromthe positive-electrode plate.

A circular negative electrode 14.0 mm in diameter was punched out fromlithium metal foil 300 μm in thickness.

Fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethylmethyl carbonate (EMC) were mixed at a volume ratio of 1:1:6 to preparea non-aqueous solvent.

LiPF₆ was dissolved at a concentration of 1.0 mol/liter in thenon-aqueous solvent to prepare a non-aqueous electrolyte solution.

A separator (manufactured by Celgard, LLC., product number 2320, 25 μmin thickness) was impregnated with the non-aqueous electrolyte solution.

Celgard (registered trademark) 2320 is a 3-layer separator composed of apolypropylene layer, a polyethylene layer, and a polypropylene layer.

A CR2032 coin-type battery was assembled from the positive electrode,the negative electrode, and the separator in a dry box maintained at adew point of −50° C.

Examples 2 to 5

The Li/A/Me ratio and the O/F ratio were changed from those described inExample 1.

Table lists the precursors for the production of positive-electrodeactive materials of Examples 2 to 5 and the component ratios of thepositive-electrode active materials thus synthesized.

Except for these, the positive-electrode active materials of Examples 2to 5 were synthesized in the same manner as in Example 1. The precursorsof Examples 2 to 5 were weighed at the stoichiometric ratio and weremixed in the same manner as in Example 1.

Coin-type batteries were produced in the same manner as in Example 1 byusing the positive-electrode active materials of Examples 2 to 5.

Example 6

Lithium cobalt oxide (LiCoO₂) was produced by a known method.

The lithium cobalt oxide, LiF, and NaF were weighed at a mole ratio ofLiCoO₂/LiF/NaF=1.0/0.9/0.1.

The raw materials, together with a proper amount of 3-mm zirconia balls,were put in a 45-cc zirconia container, which was then sealed in anargon glove box.

It was removed from the argon glove box and was treated in a planetaryball mill at 600 rpm for 30 hours.

The resulting compound was subjected to X-ray powder diffractionmeasurement.

The space group of the compound was FM3-M.

The composition of the compound was determined by ICP spectroscopy andinert gas fusion-infrared absorption.

The composition of the compound was Li_(1.9)Na_(0.1)CoO₂F.

A coin-type battery was produced in the same manner as in Example 1 byusing the positive-electrode active material of Example 6.

Comparative Example 1

LiF, Li₂O, and Mn₂O₃ were weighed at a mole ratio ofLiF/Li₂O/Mn₂O₃=1.0/0.5/0.5.

The raw materials, together with a proper amount of 3-mm zirconia balls,were put in a 45-cc zirconia container, which was then sealed in anargon glove box.

It was removed from the argon glove box and was treated in a planetaryball mill at 600 rpm for 30 hours.

The resulting compound was subjected to X-ray powder diffractionmeasurement.

The space group of the compound was FM3-M.

The composition of the compound was determined by ICP spectroscopy andinert gas fusion-infrared absorption.

The composition of the compound was Li₂MnO₂F.

A coin-type battery was produced in the same manner as in Example 1 byusing the positive-electrode active material of Comparative Example 1.

Comparative Example 2

Lithium cobalt oxide (LiCoO₂) was produced by a known method.

The space group of the lithium cobalt oxide was R3-M.

LiF and LiCoO₂ were weighed at a mole ratio of LiF/LiCoO₂=1.0/1.0.

The raw materials, together with a proper amount of 3-mm zirconia balls,were put in a 45-cc zirconia container, which was then sealed in anargon glove box.

It was removed from the argon glove box and was treated in a planetaryball mill at 600 rpm for 30 hours.

The resulting compound was subjected to X-ray powder diffractionmeasurement.

The space group of the compound was FM3-M.

The composition of the compound was determined by ICP spectroscopy andinert gas fusion-infrared absorption.

The composition of the compound was Li₂CoO₂F.

A coin-type battery was produced in the same manner as in Example 1 byusing the positive-electrode active material of Comparative Example 2.

<Evaluation of Battery>

The electric current density on the positive electrode was set at 0.05mA/cm², and the battery of Example 1 was charged to a voltage of 5.2 V.

Subsequently, the discharge cut-off voltage was set at 1.5 V, and thebattery of Example 1 was discharged at an electric current density of0.05 mA/cm².

The initial discharge energy density was 1080 mWh/g.

The initial discharge energy density of the coin-type battery ofComparative Example 1 was measured in the same manner as in Example 1.

The initial discharge energy density of the battery of ComparativeExample 1 was 1060 mWh/g.

The electric current density on the positive electrode was set at 0.05mA/cm², and the battery of Comparative Example 2 was charged to avoltage of 5.2 V.

Subsequently, the discharge cut-off voltage was set at 2.5 V, and thebattery of Comparative Example 2 was discharged at an electric currentdensity of 0.05 mA/cm².

The initial discharge energy density of the battery of ComparativeExample 2 was 990 mWh/g.

The energy densities of the coin-type batteries of Examples 2 to 5 weremeasured in the same manner as in Example 1.

Table shows the results.

TABLE Space Energy density Sample Precursor Composition group mWh/gExample 1 LiF—NaF—Li₂O—Mn₂O₃ Li_(1.9)Na_(0.1)MnO₂F FM3-M 1080 Example 2LiF—NaF—Li₂O—Mn₂O₃ Li_(1.95)Na_(0.05)MnO₂F FM3-M 1075 Example 3LiF—NaF—Li₂O—Mn₂O₃ Li_(1.99)Na_(0.01)MnO₂F FM3-M 1066 Example 4LiF—NaF—Li₂O—Mn₂O₃ Li_(1.6)Na_(0.2)MnO₂F FM3-M 1065 Example 5LiF—KF—Li₂O—Mn₂O₃ Li_(1.95)K_(0.05)MnO₂F FM3-M 1066 Example 6LiF—NaF—LiCoO₂ Li_(1.9)Na_(0.1)CoO₂F FM3-M 1004 ComparativeLiF—Li₂O—Mn₂O₃ Li₂MnO₂F FM3-M 1060 example 1 Comparative LiF—LiCoO₂Li₂CoO₂F FM3-M 990 example 2

Table shows that the initial discharge energy density of the batteriesof Examples 1 to 5 ranged from 1065 to 1080 mWh/g.

The energy densities of the batteries of Examples 1 to 5 were largerthan that of Comparative Example 1.

The plausible reason for the larger energy densities in Examples 1 to 5is that an alkali metal with a large ionic radius in the crystalstructure distorted the lattice and thereby improved the diffusion ofLi.

Table also shows that the energy density of the battery of Example 2 issmaller than that of Example 1.

The plausible reason for the smaller initial energy density in Example 2is that a smaller amount of Na in the solid solution resulted in smallerlattice distortion and slower diffusion of Li.

Table also shows that the initial discharge energy density of thebattery of Example 3 is smaller than that of Example 1.

The plausible reason for the smaller energy density in Example 3 is thata smaller amount of Na in the solid solution resulted in a smallereffect of lattice distortion due to Na solid solution.

Table also shows that the energy density of the battery of Example 4 issmaller than that of Example 1.

The plausible reason for the smaller energy density in Example 4 is thata larger amount of Na in the solid solution resulted in slower diffusionof Li and a smaller amount of Li involved in the reaction.

Table also shows that the energy density of the battery of Example 5 issmaller than that of Example 1.

The plausible reason for the smaller energy density in Example 5 is thatsolid solution of K with a large ionic radius increased theoxygen-oxygen distance in a first vicinity of K, and a decrease in theoxygen-oxygen distance in a second vicinity to reduce distortionretarded the diffusion of Li.

Table also shows that the energy density in Example 6 is larger thanthat of Comparative Example 2.

The plausible reason for the larger energy density is that as in Example1 solid solution of an alkali metal with a large ionic radius causedlattice distortion and improved the diffusion of Li.

Thus, whether Me denotes the element or the solid solution containingthe element as described above, the addition of the alkali metal canincrease the energy density.

As shown in Table, y of 0 in the composition formula (1) (for example,y=0 in Comparative Example 1) resulted in no distortion due to Na solidsolution. This resulted in decreased energy density.

Also as shown in Table, y of more than 0.1 (for example, y=0.2 inExample 4) resulted in slower diffusion of Li due to Na and a decreasedamount of Li involved in the reaction. Thus, the energy densitydecreased.

The results show that satisfying 0<y≦0.1 can further increase the energydensity.

A positive-electrode active material according to the present disclosurecan be suitable for a positive-electrode active material of batteries,such as secondary batteries.

What is claimed is:
 1. A positive-electrode active material comprising acompound that has a crystal structure belonging to a space group FM3-Mand is represented by the composition formula (1):Li_(x)A_(y)Me_(z)O_(α)F_(β)  (1) wherein A denotes Na or K, Me denotesone or two or more elements selected from the group consisting of Mn,Co, Ni, Fe, Al, B, Ce, Si, Zr, Nb, Pr, Ti, W, Ge, Mo, Sn, Bi, Cu, Mg,Ca, Ba, Sr, Y, Zn, Ga, Er, La, Sm, Yb, V, and Cr, and the followingconditions are satisfied.1.7≦x+y≦2.20≦y≦0.20.8≦z≦1.31≦α≦2.50.5≦β≦2
 2. The positive-electrode active material according to claim 1,containing the compound as a main component.
 3. The positive-electrodeactive material according to claim 1, satisfying x+y+z=α+β=3.
 4. Thepositive-electrode active material according to claim 1, wherein Medenotes one element selected from Mn and Co.
 5. The positive-electrodeactive material according to claim 1, satisfying 0<y≦0.1.
 6. Thepositive-electrode active material according to claim 1, satisfying1.8≦x≦1.99, and0.01≦y≦0.2.
 7. The positive-electrode active material according to claim1, satisfying z=1, α=2, and β=1.
 8. A battery comprising: a positiveelectrode containing the positive-electrode active material according toclaim 1; a negative electrode; and an electrolyte.
 9. The batteryaccording to claim 8, wherein the negative electrode contains anegative-electrode active material with lithium adsorption anddesorption characteristics, and the electrolyte is a non-aqueouselectrolyte solution.